An active filter is a type of analog electronic filter distinguished by the use of one or more active components. An active low pass filter may be formed by connecting in parallel a feedback resistor across the respective input and output terminals of an operational amplifier. Additionally, a capacitor is connected in parallel to the feedback resistor.
Active analog filters used in RF devices are typically implemented on-chip. Often, capacitors occupy substantial portion of the die area compared to other components typically present in active analog filters. To minimize die area, it is desirable to make capacitor dimensions as small as possible. In state of the art highly integrated CMOS processes, strong design constraints exist that prevent area reduction of a circuit especially where passive components, such as capacitors, dominate the die area.
FIG. 1 shows an active analog filter 100 configured as a low pass filter. Active analog filter 100 includes operational amplifier 120, feedback resistor 110 and capacitor 130. Feedback resistor 110 is connected across operational amplifier 120 from the Vout output terminal to the Iin negative input terminal. Capacitor 130 is connected in parallel to feedback resistor Rf 110. A corner frequency, F−3dB of the filter is set, in principle, by the product of the resistance (Rf) of resistor 110 times the capacitance (Cf) of capacitor 130 as shown in the following equation:
                              F          -                      3            ⁢                                                  ⁢            dB                          =                  1                      Cf            *            Rf                                              Eq        .                                  ⁢                  (          1          )                    
Either (i) Metal-Insulator-Metal (MIM) capacitors or (ii) Metal-Oxide-Semiconductor (MOS) capacitors may be used for the implementation of integrated active analog filters, especially for modern wireless telecommunication devices.
A MIM capacitor is a particular type of capacitor having two metal plates facing each other across capacitor dielectrics. MIM capacitors are known to exhibit high linearity over broad voltage ranges, low series resistance, small temperature coefficients of capacitance, low leakage currents, and sufficient dielectric reliability. MIM capacitors, however, are rather large compared, for example, to transistors or memory cells. Despite their relatively large size, MIM capacitors are typically preferred in cases where linear capacitance over voltage range is required.
MOS capacitors are preferable, in cases where circuit linearity is not critical, because they occupy less die area. In a typical 65 nm CMOS process, MOS capacitors require less than a quarter of the space that MIM capacitors require. A MOS capacitor is a well known type of capacitor device built on top of an n-well diffusion lying over a p substrate.
FIG. 2 shows a typical MOS varactor. MOS capacitors are often implemented as MOS varactors. Gate 210 and the two n+ contacts 220, 230 inside n-well 240 form two plates of capacitor. MOS varactors utilize both the depletion and accumulation operation regions of an MOS device thus offer higher capacitance per unit area as an NMOS channel is formed even when the potential between gate and source is zero.
One disadvantage of MOS capacitors, and equally MOS varactors, is poor capacitance linearity performance over voltage range. This is especially true in the presence of large signals across active analog filters.
A typical capacitance over voltage curve (C-V curve) of a MOS varactor may be modeled as ‘β*tan h(α·Vgs)’ where α and β are parameters particular to the selected process technology. Vgs is the voltage across gate terminal 210, and source terminal 220, of MOS varactor 200, as illustrated in FIG. 2.
FIG. 3 shows a C-V curve 300 of a typical MOS varactor. C-V curve 300 is highly non-linear over its voltage operating range. As a result, MOS varactors are unfit for applications requiring high linearity, such as active analog filters. An active analog filter having a MOS capacitor device with improved linearity is desirable.